WARNING: Untested, incomplete code ahead

I would create this script in form of a filter:

#!/usr/bin/perl -i.bak
  :
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and would make the file_a an option so that you can do it like this:

$ ./myfilter -i file_a file_b
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This would then create a new file_b and preserve a backup file_b.bak.

First thing you should do in the filter is find all the things you'd like to replace in file_b (should it be more than just one) and store it in a hash:

while (<filea>) {
    if ( this is a line i want ) {
        s/[\015\012]+$//; # Just to remove any CR/LF
        my($router,$site)= split /,/,$_,3;
        $replace{$router}= $site;
    }
}
my $replacements= '\b('.join('|',keys %router).')\b';
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After that you have a list of texts to search for as keys %replace and the replacement as $replace{...}. You also have a string of the form: \b(router1|router2...|routerN)\b that will be used in a regular expression.

When you've done with that, simply read and write what ever comes through <> replacing what you need:

my $symbol;
while (<>) {
    if (my $hit/^SYMBOL: $replacements/i ... /^END SYMBOL/i) {
        if ($hit==1) {
            $symbol=$1;
        }
        else {
            s/^(Parent Submap:).*/$1 $replace{$symbol}/;
        }
    }
    print;
}
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That's it.